Conventionally, 2-mercapto benzothiazole (MBT) based derivatives such as Benzothiazyl disulfide (MBTS), N-cyclohexylbenzothiazole-2-sulfenamide (CBS), N-tert-butylbenzothiazole-2-sulfenamide (TBBS), N-oxydiethylenebenzothiazole-2-sulfenamide(MBS) and N,N′-dicyclohexylbenzothiazole-2-sulfenamide(DCBS) are used as accelerators for the sulfur vulcanization of various tire compounds based on Natural Rubber(NR), Styrene-Butadiene Rubber(SBR), Polybutadiene Rubber (BR) or their blends. Under the conditions of high temperature cure deployed for increased productivity of smaller size tyres or longer cure cycles used for relatively thick cross section larger diameter heavy duty tires; these accelerators are known to cause relatively faster ‘Reversion’ (degradation of polysulfide cross links) and exhibit ‘Rapid Changes’ in the vulcanizate properties (e.g. Tensile strength, Modulus at lower extensions, Hardness, Tear strength, Flex-fatigue properties and Chipping and chunking resistance etc.) during tire cure, storage and the subsequent service life of the tire due to random scission of network chains, chemical modification of polymer chains, changes in cross link character and cross link density, etc.
The increased Modulus and Hardness on ageing/during tire service affect the tire performance with respect to ‘Road Grip’ [Drive Safety!], ‘Flex-Fatigue Resistance’, Tread wear, besides affecting the “Feel” of a New Passenger Car Tire based on Synthetic Rubbers; while the overall performance, ‘Chipping and Chunking Resistance’ and the Expected Service Life of Truck & Bus Tires, Off The Road Tires (which are predominantly based on Natural Rubber) are seriously affected due to the effects of ‘Reversion’ and Rapid changes in the Sulfur Cross Link Types and the Cross Link Density'.
The use of 4m-MBTS [2,2′-dithiobis(4-methyl benzothiazole)] derivative of Na salt of 4m-MBT (4-Methyl-2-Mercaptobenzothiazole) as a Modulus Stabilizing Accelerator for sulphur cured tire compounds has been described in European Patent Application EP 0 945 482 A1.
U.S. Pat. No. 5,840,908 describes mono- and bisubstituted benzothiazole sulfenamide compounds based on primary amines may be employed as rubber vulcanization accelerators having excellent scorch safety. Vulcanizable rubber compositions containing rubber, sulfur and the benzothiazole sulfenamide compounds are also provided.
In practice, sulfur vulcanized rubber compounds suffer from the adverse effect of Reversion leading to high hysteresis and subsequently higher heat generation. Heat generation causes rapid tread wear of large truck and off-the-road tyres. In addition to this the tire tread surface gets subjected to longer curing times and hence reversion, to ensure optimum state of cures requirements of the inner sections of the tires. To combat these detrimental effects of reversion, the curing systems employed are with high accelerator/sulfur ratios and thereby improving the reversion resistance since the resulting network is based on more thermally stable cross links. The cross link network formed during the vulcanization process is predominantly polysulphidic and tends to degrade into di & monosulfidic cross links under the influence of heat supplied during prolonged tire vulcanization process, subsequent tire storage and the heat generated during the service of the tire.
These unavoidable circumstances cause rapid changes in the nature & density of cross link matrix affecting the originally designed stress-strain & other vulcanizate properties of the various rubber components when conventional sulfenamide accelerators are used in the tire compounds.
Therefore, there remains a need in the art to develop a novel sulfenamide accelerator as a solution for overcoming the above drawbacks of conventional sulfenamide accelerators and at the same time without any negative effect on the processing, vulcanizing as well as the vulcanizate & performance properties of the tire compounds.
Thus the inventors of the present invention endeavour the shortcomings of the prior art by incorporating the novel sulfenamide derivatives having substitution at 4-position with methyl group as accelerators of sulfur vulcanization and thereby stabilizing the vulcanizate network and provide improved ‘Reversion Resistance’ to tire compounds predominantly based on Natural Rubber or its blends with Polybutadiene (BR) & Styrene Butadiene (SBR) Rubbers in which Natural rubber is a major component.
These novel sulfenamide accelerators also offer the advantage of ‘Modulus and Hardness Stabilization’ for sulphur vulcanized tire compounds predominantly based on Natural Rubber or its blends with general purpose synthetic rubbers like BR & SBR and it is expected that the feel of a new tire, its overall performance on the road, ride safety and tire service life will be improved considerably.